1. Field of the Invention
The present invention relates to a vinylpyrrolidone-based copolymer having as essential components N-vinylpyrrolidone and hydroxyl group-containing (meth)acrylate, such as 2-hydroxyethyl (meth)acrylate, and a method for production thereof.
2. Description of the Related Art
It is known that a copolymer of N-vinylpyrrolidone and 2-hydroxyethyl (meth)acrylate (occasionally referred to as “NVP/HE(M)A copolymer” hereinafter) is useful as a raw material polymer of a curable resin composition, which allows a film having a water resistance and hydrophilicity together because a structure thereof has both a hydrophilic pyrrolidone group and a hydroxyl group reactable with a crosslinking agent such as isocyanate (see Japanese Patent Laid-open (Kokai) Publication No. 2003-113220). The copolymer is utilized also as a preferable polymer in a curable resin composition (see Japanese Patent Laid-open (Kokai) Publication No. 2003-286381) containing as essential components a polymer having an N-vinylamide unit, a compound having two or more functional groups for reacting with active hydrogen, and a polyvalent metallic compound. The application of the copolymer to a semiconductor cleaning agent and a protective film is expected because the copolymer has properties such as high adsorptive property, water-solubility and film-forming ability.
The NVP/HE(M)A copolymer is conventionally obtained by adding 2-hydroxyethyl (meth)acrylate to water (solvent) containing N-vinylpyrrolidone and a polymerization initiator and carrying out the polymerization, and the process is generally employed as a process for production of the NVP/HE(M)A copolymer.
However, in the case where a curable resin composition employing the NVP/HE(M)A copolymer as a raw material polymer is formed into a film and crosslinked, the copolymer obtained by a conventional technique causes a problem in which the surface smoothness of the formed crosslinked coating film is deteriorated, and gel is swollen during washing with water for removing the unreacted raw material, and the like, after crosslinking as well as a problem in which the uniformity of the formed crosslinked coating film is lowered. Another problems in which the crosslinked coating film employing the conventional NVP/HE(M)A copolymer as a raw material polymer has a low water resistance arise.
The above mentioned problems are problems common to a vinylpyrrolidone-based copolymer employing hydroxyl group-containing (meth)acrylate typified by 2-hydroxyethyl (meth)acrylate as the other raw material monomer.
The NVP/HE(M)A copolymer obtained by the conventional technique is high in the content of an ionic component such as metal, and some applications of the copolymer cause a problem in electrical conductivity, coloring, and the like, so that it is also desired that the content of the ionic component such as metal in the NVP/HE(M)A copolymer be decreased.
In addition, an NVP homopolymer having no hydroxyl groups with a high reactivity is occasionally contained in a large amount in the NVP/HE(M)A copolymer obtained by the conventional technique. In this case, the problem in which an uncrosslinked portion is made in the coating film and the water resistance of the coating film is deteriorated in forming the crosslinked coating film arises. Under such circumstances, the NVP/HE(M)A copolymer having a low NVP homopolymer content is desired.
Therefore, the problem to be solved by the present invention is to provide a vinylpyrrolidone-based copolymer with a low metal content having as essential components N-vinylpyrrolidone and hydroxyl group-containing (meth)acrylate, such as 2-hydroxyethyl (meth)acrylate, which gives a favorable crosslinked coating film without causing the above-mentioned problems, and a method for production by which the copolymer is easily obtained.